1. Field of the Invention
The present invention relates to integrated circuits, and more particularly to electromagnetically shielded silicon-on-insulator (SOI) sensors and similar electronic devices.
2. Description of the Related Art
Integrated circuits generally comprise electronic devices and electrical interconnections formed in or on a thin layer of semiconductor material. Electromagnetic and electrostatic fields affect the performance of the electronic devices. For example, nearby circuitry can generate an electric field in the integrated circuit and can move free electrons across the active semiconductor layer. This electric field changes charge distribution in the semiconductor material that varies as the strength of the electric field changes.
A silicon-on-insulator (SOI) device includes an active semiconductor layer, such as silicon or the like, over an insulator layer. Generally, the insulator layer comprises a buried oxide (BOX) region of a substrate which provides structural support to the device. The BOX region is generally formed below a top surface of the substrate, and the active semiconductor layer is formed over the BOX region. The active semiconductor layer includes circuit elements such as resistors, transistors, diodes, or the like. The performance of the circuit elements formed in the active semiconductor layer can be affected by electromagnetic or electrostatic fields.
SOI devices can operate at high temperatures and are used to create high-speed integrated circuits and sensors to measure, for example, pressure, acceleration, temperature, or the like. An SOI sensor that measures mechanical displacement includes a piezoresistive strain gauge formed in the active semiconductor layer which is isolated from the underlying substrate by the insulation layer. Generally, the active semiconductor layer comprises doped monocrystalline silicon, polycrystalline silicon, germanium, or composite Ill-V class semiconductors. The SOI sensor also has a flexible substrate and the resistance of the strain gauge varies with mechanical displacement of the flexible substrate as pressure or force on the SOI sensor causes small mechanical deformations that stretch or compress the active semiconductor layer.
The strain gauge may include, for example, a network of resistors in a bridge circuit. Pressure variation changes the resistances of the bridge circuit by mechanically deforming the SOI sensor, enabling the resistors of the bridge circuit to generate a differential voltage across the bridge proportional to the measured pressure. The accuracy of pressure measurements is affected by the stability of the resistor values as a function of flexing the crystal grid of the active semiconductor material.
Factors such as high temperatures, electromagnetic interference, or strong static fields can affect the piezoelectric resistor values or other circuitry and introduce error into the pressure measurements. For example, the presence of stray electric fields induces field effects in the active semiconductor material that effectively create channels that allow current to flow more easily (reducing resistivity) or that pinch off the current (increasing resistivity) as the fields change. In some applications, oil with a high dielectric value is used to isolate the SOI device from a hostile environment, such as inside an engine or fuel tank. Generally, such oils exhibit a dipole moment when heated. The dipoles of the oil produce an electric field potential in close proximity to the circuit elements in the active semiconductor layer which causes drift currents resulting in measurement error. Other SOI devices used in systems which generate stray electric fields or static buildup within the SOI device itself or within its packaging may also suffer from such error.